How can you find the credibility of the author when doing an annotated bibliography?
Many small pieces of information can help you assess an author's credibility for an annotated bibliography. Look or search for the author's credentials/affiliations (i.e., university professor? where? in what field or department--is it one relevant to the article?, etc), previous work (have they written on this topic before?), or any other biographical information you can find. You might need to do a separate Google search to find this type of information, or if you're working in a database, you might find what you need in the article record (the page with the abstract, title, page numbers, etc).
If you are trying to evaluate a source where the author is not listed by name or you can't find information about the individual writer, you can also consider the authority/credibility of the organization publishing the information.